Transparent conductors are widely used in the flat-panel display industry to form electrodes for electrically switching the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square).
Conventional transparent conductors are typically coated on a substrate to form a patterned layer of a transparent, conductive material, such as indium tin oxide or other metal oxide. Such materials are increasingly expensive and relatively costly to deposit and pattern. Moreover, metal oxides have a limited conductivity and transparency, and tend to crack when formed on flexible substrates or when curved. Conductive polymers are also known, for example polyethylene dioxythiophene (PEDOT). However, such conductors have a relatively low conductivity and transparency.
More recently, transparent electrodes including very fine patterns of conductive micro-wires have been proposed. For example, capacitive touch-screens with mesh electrodes including very fine patterns of conductive elements, such as metal wires or conductive traces, are taught in U.S. Patent Application Publication No. 2010/0328248 and U.S. Pat. No. 8,179,381, which are hereby incorporated in their entirety by reference. As disclosed in U.S. Pat. No. 8,179,381, fine conductor patterns are made by one of several processes, including laser-cured masking, inkjet printing, gravure printing, micro-replication, and micro-contact printing. The transparent micro-wire electrodes include micro-wires between 0.5μ and 4μ wide and a transparency of between approximately 86% and 96%.
Conductive micro-wires are formed in micro-channels embossed in a substrate, for example as taught in CN102063951, which is hereby incorporated by reference in its entirety. As discussed in CN102063951, a pattern of micro-channels is formed in a substrate using an embossing technique. Embossing methods are generally known in the prior art and typically include coating a curable liquid, such as a polymer, onto a rigid substrate. The polymer is partially cured (through heat or exposure to light or ultraviolet radiation) and then a pattern of micro-channels is embossed (impressed) onto the partially cured polymer layer by a master having a reverse pattern of ridges formed on its surface. The polymer is then completely cured. A conductive ink is then coated over the substrate and into the micro-channels, the excess conductive ink between micro-channels is removed, for example by mechanical buffing, patterned chemical electrolysis, or patterned chemical corrosion. Metal nano-particle compositions are known, for example as disclosed in U.S. Patent Application Publication No. 2011/0303885. The conductive ink in the micro-channels is cured, for example by heating. In an alternative method described in CN102063951, a photosensitive layer, chemical plating, or sputtering is used to pattern conductors, for example using patterned radiation exposure or physical masks. Unwanted material (photosensitive resist) is removed, followed by electro-deposition of metallic ions in a bath.
It is useful to form many electronic devices on flexible substrates. Flexible substrates are robust in the presence of mechanical shock and enable a wide variety of useful end-product form factors that are not readily achieved with electronic devices formed on rigid substrates. In particular applications, electronic devices are formed on flexible substrates in a flat configuration and then the electronic devices and flexible substrates are bent or otherwise mechanically manipulated to form non-planar shapes, for example a cylindrical shape or portion of a cylindrical shape. Since most electronic fabrication processes rely on flat substrates, the ability to form electronic devices in a flat configuration and then bend or curve the electronic device permits conventional manufacturing equipment designed for conventionally rigid and flat substrates to be used for making devices that are ultimately used in non-flat arrangements.
Polymer layers are used to conduct light in channels formed in a substrate, for example as disclosed in U.S. Pat. No. 7,371,452. Resins used for blocking light and formed in channels are discussed in U.S. Pat. No. 8,269,404. However, these disclosures do not provide conductive micro-wires.
In useful arrangements, conductive micro-wires are used in electronic devices to form apparently transparent electrodes or to provide conductors in electronic circuits. There is a need, therefore, for robust and manufacturable micro-wire structures that enable improved conductivity in non-flat configurations.